An integrated-optic (IO) geometry for active waveguide devices possesses several desirable features. This device allows active components to be combined with passive components on a single substrate, resulting in a reduction in device size and cost. Furthermore, higher rare earth (RE) dopant concentrations may be possible in IO waveguides than in conventional doped optical fibers because in some chalcogenide as well as non chalcogenide glasses the presence of RE ions may increase the tendency towards crystallization during the fiber draw process. Crystallization can lead to high propagation losses or even breakage of the fiber.
In IO waveguides, any RE dopant concentration is possible in principle as long as the composition lies within the glass-forming region. As a result, active IO devices have the potential for high gain per unit length. Therefore, optical amplification can be accomplished in a shorter distance resulting in more compact and lower cost devices for applications such as telecommunications and gas sensing.
The chalcogenide glasses (ChG's) can be good candidates for use in RE doped amplifiers and lasers operating at wavelengths from the telecommunications bands through the long-wave IR. These glasses exhibit low phonon energies, resulting in transparency in the mid wave and long wave IR spectral regions. Furthermore, these glasses permit transitions that are not possible in silica and phosphate glasses due to multiphonon quenching. ChG's have been used previously in the fabrication of fiber lasers as well as in optically written IO lasers. They have not, however, been previously used in erbium-doped waveguide amplifiers (EDWA's).
Gallium lanthanum sulfide (GLS) is a type of chalcogenide glass that may be particularly well-suited for use in EDWA's. It has a wide IR transmission window, with transmission of 50% or higher through a 1 mm thickness over a wavelength range of 0.5 to 10 μm. Its high glass transition temperature relative to that of other chalcogenide glasses, Tg=580° C. makes GLS appropriate for use in high-temperature applications. Additionally, high RE dopant concentrations without clustering are possible because RE ions are able to substitute for lanthanum ions in the glass matrix. Clustering of the RE ions can lead to a reduction in amplification per unit length within the material. The potential of GLS as a laser material has been demonstrated in Nd3+-doped bulk glass lasers and Nd3+-doped fiber lasers. In addition, the spectral properties of bulk Er3+-doped GLS have been well-characterized.
Previous efforts to fabricate IO waveguides in GLS glass have utilized the fact that the glass exhibits a photoinduced refractive index change when exposed to above bandgap radiation—light with a wavelength of less than approximately 0.5 nm. Waveguides written in polished bulk samples with propagation losses of <0.5 dB/cm have been achieved. However, forming a waveguide in a thin film of sputtered GLS, possess several advantages over waveguides written in polished bulk glass. A sputtered film can be made with high compositional uniformity and with fewer of the local defects that can be present in bulk glass. In addition, deposition results in improved design flexibility, the sputtered film can be prepared on a plurality of substrates, thus facilitating integration with other IO devices. Further, the glass is deposited with greater precision and thereby potentially reducing cost. In addition, previous efforts have included fiber based optical amplifiers that are operable in the telecommunications arena. While these fiber-based amplifiers have become a key technology for long haul communications networks, where transmission distances are typically >300 km. they are bulky and costly. There is a need for smaller, lower cost amplifiers with less demanding performance requirements that can be easily installed in smaller networks. IO waveguide amplifiers can satisfy these requirements.
Optical losses are reduced as a result of improved compositional uniformity and a lack of the local defects that are present in glass melts. The ability to deposit the active glass only where it is needed leads to more flexibility in device design, compatibility with a variety of substrate materials, and more straightforward integration with other IO devices. Cost is reduced because the active glass is deposited only where needed, so a smaller volume is required than for a bulk sample.